1. Field of the Invention
This invention relates to Universal Serial Bus (USB) 2.0 point-to-point protocols, and particularly to a system and method for allowing a designer to implement USB 2.0 in topologies not anticipated by the USB 2.0 specification and without channel losses that would cause the received signal to fail the USB receive eye mask.
2. Description of Background
Universal Serial Bus (USB) 2.0 is a complete overhaul of the Universal Serial Bus input/output bus protocol, which allows much higher speeds than the older USB 1.1 standard did. The goal of the USB 2.0 serial bus is to broaden the range of external peripherals that can be used on a computer. A hard drive can easily hit the USB 1.1 bottleneck whereas it now becomes more usable under USB 2.0 conditions.
USB 2.0 is a point-to-point protocol that makes certain assumptions about physical interconnect of the two agents. The two agents are typically connected through a cable with only minimal amounts of trace on a printed circuit board (PCB). In an environment such as the BladeCenter® chassis it is desirable to drive USB 2.0 over many inches of FR4 (24″) with multiple passive Field Effect Transistor (FET) switches and connectors in the path. Because of longer distances through FR4 and because of the intervening devices in the path, the resulting transmission channel is very lossy and likely does not meet the required USB receiver eye mask.
IBM BladeCenter® offers a broad range of storage and networking options integrated into the chassis to simplify infrastructure complexity and manageability while lowering total cost of ownership. IBM BladeCenter® supports a wide selection of processor technologies and operating systems to allow clients to run all of their diverse work loads inside a single architecture. Reducing complexity, improving systems management, and increasing flexibility while driving down total cost of ownership are key solutions. FR-4, an abbreviation for Flame Resistant 4, is a type of material used for making a PCB. It describes the board itself with no copper covering.
There are some solutions available to solve the problem of a lossy channel. Some of these solutions include: 1) Adding a USB hub to the middle of the channel. This would require adding a complex Application Specific Integrated Circuit (ASIC) and associated circuitry to the backplane. 2) Incorporating pre-emphasis to USB transmitters and equalization (EQ) to USB receivers. Neither pre-emphasis nor EQ are specified in the USB 2.0 and both endpoints must support these features, thus the user of these features would have limited operability. 3) Transmission line matching network. This is typically only useful to combat specific discontinuities. Using passive filter networks to compensate for FR4 loss usually requires a boost in transmit voltage to overcome the loss in the filter. USB 2.0 does not specify a variable transmit voltage. 4) Installing USB range extenders. These devices are conceptually USB hubs with a non-USB interconnect in between them and have the same disadvantages as USB hubs. Therefore, these four solutions are not effective in dealing with the problem of lossy channels.
Considering the above limitations, it is desired to have a method for allowing a designer to implement USB 2.0 in topologies not anticipated by the USB 2.0 specification and without channel losses that would cause the received signal to fail the USB receive eye mask.